With advances in Internet of Thing (IoT) applications and the expansion of mobile devices, energy consumption has become a primary focus of attention in integrated circuits design. These mobile battery operated devices need to operate for extended periods without recharging and therefore requiring ultra-low energy consumption. Many IoT devices require operation in a wide range of frequencies that are dynamically defined by the application. Low voltage operation in the “near-threshold” region has been shown to be the ideal way to dramatically reduce energy dissipation, still achieving reasonable performance. However, an aggressive scaling of supply voltage results in performance degradation and a much higher sensitivity to process variations and temperature fluctuations.
In addition to the reduction in supply voltage, many of the circuits are duty cycled, and turned off during sleep states. However, there are several types of circuits which need to be “always-on” and operate during standby mode. Among these circuits are real-time-clocks (RTC) and power management circuits, such as low drop out regulators (LDO) and DC-to-DC converters. All of these always-on elements require analog voltage and current references. To meet these requirements, there has been significant recent interest in ultra-low power references.
One such reference known in the prior art is the so-called 2T (two terminal) transistor-based voltage reference, which uses two MOSFET transistors sized such that the temperature coefficients of their threshold voltages (Vth) cancel out, thereby yielding a voltage reference which is temperature independent. Another 2T version uses native zero threshold devices which can produce a reference voltage independent of Vdd with only two transistors. Although the 2T references are very attractive due to their simplicity and ultra-low power (pW range), they have not yet found acceptance in most IOT systems. This is because the temperature coefficient of Vth is not necessarily guaranteed by the process, especially in advanced nodes. In general the use of the temperature dependence of Vth in MOS devices is not considered reliable in real products, since it can change over the course of the product lifetime, due to speed up of the process.
Many computer systems utilize reference voltages produced by the parasitic Bipolar Junction Transistor (BJT), a.k.a. diode based references. The most common of these is shown in FIG. 1 which is a sub-bandgap reference. To first order, the current and voltage across the BJT are as follows:
                              I          C                =                                            I              S                        ⁢            exp            ⁢                                                  ⁢                          (                                                qV                  be                                KT                            )                        ⁢                                                  ⁢            and            ⁢                                                  ⁢                          V              BE                                =                                    V                              g                ⁢                                                                  ⁢                00                                      -                          λ              ⁢                                                          ⁢              T                                                          [        1        ]            
where Ic is the collector current, Vbe is the base-emitter voltage, Vg00 is the extrapolated Vbe at 0K, K=Boltzmann constant, q=electron charge, λ is its linear temperature coefficient and T is the absolute temperature. Using equation 1, the CTAT (complimentary to absolute temperature) and PTAT (proportional to absolute temperature) terms can be calculated for the circuit in FIG. 1 to be:
                              V          ref                =                  R          ⁢                                          ⁢                      3            ⁡                          [                                                                    KT                                          q                      *                      R                      ⁢                                                                                          ⁢                      1                                                        ⁢                                      ln                    ⁡                                          (                      N                      )                                                                      +                                  Vbe                                      R                    ⁢                                                                                  ⁢                    2                                                              ]                                                          [        2        ]            
The utility of this circuit is that both the voltage and temperature coefficient of Vref can be trimmed by digitally adjusting R3 and R2* respectively. Note that the terms PN diode and BJT are used interchangeably throughout the specification and claims. Essentially, the PN Junction Diode is the parasitic PNP BJT in the CMOS process whose base and collector are both connected to Vss (or ground).
FIG. 2 shows a prior-art switched-capacitor BGREF (bandgap voltage reference) circuit which consumes only 32 nW and operates at 0.5V. The two Vbe terms are generated by a charge pump circuit, while the delta-Vbe terms are derived using a switch capacitor addition. It is important to note that in the prior art, the switch capacitor circuits are only used to perform mathematical functions by summing up the charge in the capacitor. The Vbe and delta-Vbe voltage can be summed by the switch-cap network.
One of the limitations and disadvantages of prior art nW BGREF's is very large area, since the low currents necessitate the use of very large resistors in order to generate a significant voltage across them. Another disadvantage is that due to the low currents, the wakeup times of these references can be in the milli-second range.